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  • Single-sided & Double-sided flex circuit assembly
  • Rigid, Flex & Rigid-flex PCB assembly
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  • 100% AOI, X-RAY, ICT & FCT

Characteristics of the Flex PCB Assembly

Flex PCB assembly means the assembly of electronic components on a flexible printed circuit board. The assembly and soldering process of flex PCB is more different from that of rigid printed circuit boards. Since the flex PCB is not hard enough, if we do not use a special carrier board, we can not complete the fixing and transfer. Then it is also impossible to carry out the basic SMT processes such as printing solder paste, mounting components, and reflow soldering.

So, except for the rigid PCB assembly techniques, we need to pay special attention to the pre-treatment of flex PCBs and supportive carrier boards and tools for flex PCB assembly. 

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    Viasion - Your Premier Flex PCB Assembly Supplier

    To ensure that the flexible PCB assembly is handled according to your specifications and expectations for the best results, the experts in Viasion do their best to complete the task. Even though they already have ample experience, they continue to improve their level of knowledge and understanding of flexible PCB assembly just to be able to provide better products and services of flexible PCBA to our customers. Viasion has successfully worked with different categories of companies all over the world, and the positive reviews from our customers demonstrate that we are trustworthy and considered to be one of the best companies providing flexible PCB assembly services. Moreover, you don’t have to worry about paying too much, Viasion will provide you with affordable and high-quality flexible PCB assembly services, which is what we are highly regarded for.

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    The Main Steps of the FPC Assembly Process

    In the SMT process, the flex printed circuit board, various kinds of integrated circuits, and electronic components are all soft PCB board strips and essential raw materials. Through the flex PCB SMT process, we apply these raw materials as well as integrated circuits and electronic components to soft board and welding accurately. The flexible PCB eventually becomes communication equipment, color TV, and computer main board.

    This paper is mainly to introduce the flex circuit assembly process in Viasion, hoping to let the reader get simple but detailed information about the main steps of the flexible circuit assembly process.
    1
    First we need to print the solder paste well.

    We use a scraper to push the solder paste along the surface of the SMT stencil. When the solder paste reaches a hole in the template, the downward pressure exerted by the scraper forces the solder paste through the hole in the stencil and onto the circuit board. But we need to pay attention that we need to use pallet to hold the flexible PCB in flexible PCB manufacturing process.

    2
    Then apply the adhesive.

    This process is optional. To prevent elements mounted on the bottom surface or the bottom large integrated circuit elements from melting and falling during wave soldering or double-sided reflow welding, the soft board with double-sided assembly should be glued together with an adhesive.

    3
    Then mount the components.

    In this process, the surface mount components are picked up from the feeder and accurately mounted onto the flex PCB by an automated mounting machine. It is critical that the flex PCB assembly component are placed accurate.

    4
    We need to check the components before and after welding.

    The component should be carefully checked whether the component is well mounted and whether the position is offset before the reflow welding, especially in flex PCB assembly service.

    5
    Reflow soldering process.

    After the component is placed on the solder, the solder on the pad is melted by the flow welding process of heat convection technology to form a mechanical and electrical interconnect between the component leads and the pads.

    6
    PTH component insertion.

    For through-hole, plug-in components and some surface-mounted elements that cannot be mounted by a machine, such as some plug-in connectors, electrolytic capacitors, push-button switches, and metal end electrode elements (MELFS), it is necessary to perform manual plug-in or component insertion with automatic plug-in equipment.

    7
    Wave soldering process.

    Wave soldering is mainly used for welding through-hole components.

    8
    Clean the Flex PCB surface.

    As one of the best flex PCB assembly supplier, we always supply best quality products to our customers. We never ignore any kind of defect such as stains on surface of a assembled PCB board.

    9
    Maintenance or repairing.

    This is an off-line procedure aimed at economically repairing defective solder joints or replacing defective components.

    10
    All kinds of testing.

    Testing mainly includes ICT testing and functional testing. A functional testing is always very help to ensure good quality products. But it may be not necessary for prototype flex circuits assembly some times.

    11
    Implement quality control.

    Quality management includes quality control within the production line and product quality assurance before delivery to customers. As a trusted flex PCB assembly company, Viasion implement a lots of quality inspections in flexible PCB assembly process, including: IQC inspection for electronic components, AOI inspection, ICT test, X-ray inspection, functional test etc.

    12
    Packing and sampling inspection.

    Finally, the components are packaged and sampled for inspection after packaging to again ensure the high quality of the products to be delivered to customers. The printed circuit is the core of science and technology electronic products, and the normal operation of all functions is closely related to it. The above is just a simple description of the flex PCBA process in Viasion. If the details are extended, the process is complicated.

    Prepare before Flex PCB assembly: pre-treatment and carrier board/tools

    Regular rigid PCB assembly does not need much preparation before assembly. But for flex PCB assembly, we need to analyse the PCBs case by case and plan production in advance. Usually, a pre-treatment is required before flex circuit assembly; we need to produce tools and carrier boards to hold the flex PCBs properly, and then we can use pick and place machines to assemble components. In some cases, we can only use manual assembly if machine assembly is impossible.

    1. Pre-treatment of flex PCBs

    Flexible printed circuit boards are soft and generally not vacuum-packed when they are transported out of the factory. It is easy for them to absorb moisture in the air during transportation and storage so they need to be pre-baked before carrying out the flex PCB SMT process to slowly force out the moisture. Otherwise, under the high-temperature impact of reflow soldering, the moisture absorbed by the flex PCBs will quickly vaporize into water vapor, which is prone to cause delamination, blistering and other problems. After baking, flex PCBs should have no obvious discoloration, deformation, warping, and other defects. What’s more, they need to pass the sampling inspection by IPQC before moving on to the next production process.

    Because today the electronic industry develops so fast, electronic products become smaller and more complex. To adapt to such a trend, engineers adopt the method of SMT assembly/SMT manufacturing. When some people take a look at the printed circuit boards with components of electronic products (smartphones, computers, medical electronics, telecommunication electronics or car electric parts, etc.), they may wonder how these components are mounted on the boards. Let’s reveal the answer they are mounted by the flex PCB SMT assembly.

    When you search pictures of SMT assembly factories, you can find out the prevailing presence of advanced automatic pick & place machines. Viasion utilizes advanced equipment for flex PCB boards assembly and rigid flex PCB assembly. 

    2. Production of a special carrier board

    Based on the CAD file of the flexible printed circuit board, the positioning data of the holes of the flex PCB is obtained to manufacture a high-precision positioning template of the flexible PCB and the special carrier board. The material of the carrier board should be light and thin with the characteristics of absorbing less heat and dissipating heat fast, etc. And multiple thermal shocks will not cause warping and deformation of the carrier board. Commonly used materials of the carrier board include synthetic stone, aluminum plate, silicone plate, special high-temperature resistant magnetized steel plate, and so on.

    To summarize, in the soldering process of the flexible PCB assembly, the precise positioning and fixing of the flexible PCB is the focus. The key to fixing the flexible PCB is to make a suitable carrier board. The next link to pay attention to is the steps including pre-baking, solder paste printing, component placement, and reflow soldering. Since the SMT process of the flex PCB is more difficult to carry out than that of rigid PCB, the precise setting of process parameters is necessary. At the same time, strict production management is equally important which must ensure that the operator strictly implements every rule on the SOP. The engineers who follow up on the production line and IPQC staff should strengthen inspection, timely discover the abnormal situation of the production line, analyze the causes and take necessary measures, so that the defect rate of the SMT production line which assembles flex PCBs can be controlled within tens of PPM.

    Points for Attention in the Process of Flex PCB Assembly

    According to Viasion‘s previous experience, we are very strict with flexible PCB manufacturing and flex PCB assembly services, because they are linked to high-precision technology. So in the process of the flexible PCB assembly, we must pay great attention to some matters which are listed below.

    For the solder paste that is not used immediately, we must put it in the refrigerator and keep the temperature at 5℃-0℃, not lower than 0℃.

    In the process of the flexible printed circuit assembly, the aging of the assembly machine equipment and feeder or nozzle damage will cause the occurrence of high throwing material, and easily lead to the assembly machine’s sticking crooked, which will increase production costs and reduce production efficiency. If we are unable to replace the device, it is necessary to carefully check whether the nozzle is blocked, or damaged and whether the feeder is intact.

    Whether the process parameters of reflow soldering are reasonable or not is closely related to the welding quality of the flex PCB. Generally, two furnace temperature tests, at least one, are required to continuously improve the temperature profile to match the temperature profile of the welding product. Do not miss this link for the sake of production efficiency and cost saving.

    SMT processing has certain requirements on environment, humidity and temperature. To ensure the quality of mounting, it is necessary to ensure that the processing workshop meets the requirements of the working environment. The above is SMT processing matters needing attention, which I hope can help you.

    Common Problems and Countermeasures in the Process of Flex PCB Assembly

    First, we’re going to introduce some experience about the flex PCB assembly in Viasion, and share it with you, hoping to help you.

    • Flexible printed circuit assembly looks very complex. In fact, as long as you master some points in the process of the flexible printed circuit assembly, it is not as difficult as imagined, so we must pay attention to these points： Point 1: jig; Point 2: The process of assembly.
    • Why is point 1 a fixture? This is because if the jig is done well, the subsequent assembly process is much simpler and easier. So we must pay attention to the manufacture of the jig.
    • Next, we're going to share some actual information about Viasion’s flex PCB assembly process, flex PCB assembly are used in car stereos, DVDs, industrial electronics, medical equipments, telecommunications, automotive, and so on.

    One: Fixture

    First, we need a gusset fixture. I don’t know if you know something about the gusset tooling, but this is equivalent to some common screw tooling, which has some pins for positioning, and these pins are used to buckle printing tooling and FPC.

    Next, we are talking about tooling for printing and SMT. Generally, we choose aluminum sheet material or a special material with a high-temperature resistance to make this tool. This tool can be used for printing and assembly and can be reflow welded together. We should make this fixture according to the actual situation of the flexible PCB, mainly because most of these boards are inconsistent in thickness because they want to protect part of the line or are for design reasons.

    Therefore, we need to process and polish the joint of the tooling and the flex PCB according to the actual situation, to ensure the flatness of the soft board in printing and assembly.

    Two: The Production Process

    1. Pinprick position. Based on Viasion’s flex PCB placement experience, we need to put the pinprick tooling in place first, and then put the printing tooling against the pinprick. Then place the flexible PCB in the pin position. Generally speaking, the same positioning pin is used for printing tooling and boards. It is important that the board and tooling placed in this way can be easily aligned and can ensure that there is no deviation and consistency throughout the production process.

    But that’s not enough. We need to use high-temperature adhesive paper (or paper glue but try to stick it to the unused parts of the corner joints) to fix the soft board again. The fixed point can be determined according to the size and shape of your flex PCB. We mainly ensure that the board does not shift or stick up during the mounting process and the process of the over-welding furnace.

    2. Printing. After completing the above steps, we need to remove the printing tooling (with the soft board on the buckle) directly from the gusset tooling and put it into the printing process.

    Here we should pay attention to that the above gusset plate and printing station is the key station to prevent dirt on the boards. It is a must to wear a finger sleeve, and so on. At the same time we also need to keep the station clean, and we have to wipe the net frequently, etc. (Some wire sockets of the soft board are goldfingers).

    3. Mounting: Normal Operation

    If the same tooling is used for printing assembly and over-reflow welding, then we only need to adjust the furnace temperature. Based on Viasion’s previous production experience, it is best to set the furnace temperature to the lower limit required. This will prevent wrinkling and delamination of some boards (which are rare).

    In addition, there is another reason that the line socket and a lot of soft plates use a tin plating process, and the furnace will produce bad products due to the uneven melting phenomenon of tin. As usual, this is going to be a tricky one. If this should happen, on the one hand, we can ask the customer to change the coating, which is the most ideal way to address the problem.

    If you do not want to scrap the flex PCB, you can also use some high-temperature resistant bakelite or make some aluminum strips. The size can cover the interface of the plug line. Then let the boards with bakelite or aluminum strips pass through the furnace when the furnace’s pressure covers there, playing a role of heat absorption. This method can also solve this problem.

    If you think that it is impossible to directly use the tooling with the former process to go through the furnace because the cost of tooling is high, then it is necessary to redo the tooling of the furnace. You can make the tooling larger so that you can put many flexible PCBs at a time. However, due to the need to remove the board from the assembly tooling before the furnace, in this operation process, there may be defective products, which also need us to pay great attention to.

    In the process of furnace temperature debugging, because the temperature of the board is not good, it is recommended to use the RSS_heating/insulation/reflux temperature curve, to better control the parameters of each temperature zone. In addition, the heat impact of the temperature on the board and components are less. If the DT of parts on the board is too large, this impact is greater. This is the reason why aluminum tooling is inconvenient to go through the furnace with the board at some times, and it will increase the possibility of defects.

    Of course, there may be some other problems in production, and we will discuss them next time.

    Issues must be Paid Attention to during the Process of Flex PCB Assembly

    The Differences between Flexible Printed Circuit Board Assembly and Rigid PCBA as well as the Rigid- Flex PCBA

    In essence, ordinary SMT processing steps are similar, because the flexible circuit board, rigid-flex printed circuit board, and rigid printed circuit board all need to go through the processes such as component mounting and reflow soldering. Whereas, since there are some unique features of the flex printed circuit board and rigid-flex printed circuit board, if these additional requirements cannot be seriously performed in the course of production, great troubles will occur.

    1. The process of solder paste welding

    • Just like the course of the rigid PCB, solder paste is added to the flex PCB and the rigid-flex PCB via a steel mesh and solder press operation. Lots of SMT workers will feel annoyed by the size and fragility of the flexible PCB.
    • Different from rigid PCBs, the surfaces of flexible PCBs are uneven, which explains the reason why they need to be repaired with the aid of fixtures and positioning holes.
    • Besides, the size of flexible wiring materials is not stable, capable of extending or folding 0.001 degrees per inch responding to temperature and humidity changes. Moreover, these factors concerning extension and fold will induce the board to shift in the X and Y directions.
    • That’s the reason why flexible PCB assembly and rigid flex circuit assembly often needs smaller equipment than rigid PCB assembly.

    2. The electronic components of mounting SMT

    Based on the current trend of micromation of SMT components, the features of small components may cause several problems in the process of reflow soldering. For one thing, if the flexible lines are small, a serious problem will not occur in extension and wrinkling bringing about smaller SMT mounts or additional Mark points. For another, displacement in the mounting effect will be caused by being short of the overall flatness of the vehicle. In any event, the SMT fixture plays a leading role in maintaining the flat SMT mount surface.

    3. The course in reflow welding

    The flexible line must be dried before reflow soldering, which is an important difference between placing the components of rigid-flex PCBs. Apart from the instability of dimensions of flexible materials, they can suck in water like a sponge (gaining weight up to 3% ), which is an illustration of the relatively hygroscopic feature. Additionally, the flexible printed circuit board’s absorbing moisture will be prevented by the process of reflow soldering.

    Similarly, there is the same situation for rigid PCB, but it has a higher tolerance compared with the flexible printed circuit board. Particularly the flexible printed circuit board has to be baked by preheating from 225° to 250°F. This step must be completed quickly for one hour. In case of not baked in time, it needs to be stored in a dry room or with nitrogen storage.

    Design Considerations for the Flex PCB Assembly

    Design considerations for flex circuit assembly are equal to considering the design requirements of flexible printed circuit boards.

    The design requirements for flexible PCBs consist of the following five aspects.

    1. Prerequisites for the design of flexible PCBs

    First, the design must include several elements of printed circuit boards.

    • Number of layers
    • Thickness of the board
    • Material used
    • Surface finish
    • Size of the drill bit
    • Copper drilling

    2. Descriptive notes provided for the PCB manufacturer

    Descriptive notes on the following aspects should be provided for PCB manufacturers.

    • The type of class(Class1, Class 2, Class 3) & the type of wiring
    • Flexible copper-clad material used
    • Overlay material
    • Maximum thickness of the board
    • Minimum size of plated through holes
    • Requirements for electrical testing
    • The color of the silkscreen
    • Markings on the board such as version, the logo of the company, etc.
    • Requirements for packaging and shipping.

    3. Flexibility and bending radius

    The flexibility of a flexible PCB depends on the bending radius of the flexible material used. The bending radius is the minimum angle at which the bendable area can be bent. The number of layers and the type of material used determine the bending radius.

    4. Dimensional stability

    Dimensional stability is an extremely important factor that designers must address, especially in high-density designs. During the production process, small dimensional changes can easily occur in the printed circuit boards. In addition, these PCBs are exposed to a variety of processes and environments, including etching, plating, high pressure, high temperature, and immersion in chemicals. For designers, it becomes important to consider the above issues to ensure dimensional stability.

    5. Need for shielding

    Shielding is another important parameter to consider for flexible printed circuit boards. Various methods are implemented to incorporate shielding. Shields are used to limit the effects of EMI and ESD. They also help to keep the control impedance requirements constant.

    Designers typically implement four types of shielding.

    • Copper cladding
    • Shade line of the copper
    • EMI thermoplastic shielding film

    The following points need to be noted when designing a flexible PCB.

    • Avoid placing through-holes in the bendable area of the flexible printed circuit board. Instead, through-holes should be placed in areas that are not bendable.
    • Keep using curved routing instead of one with corners.
    • Avoid sudden changes in the width of the routing.
    • The minimum space between the copper ring and the nearest through-hole should be 20 mils.
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    Frequently Asked Questions

    Generally speaking, flex circuit assembly is more complex than regular rigid PCB assembly; since the flexible circuit boards are bendable, it is difficult to run through the SMT pick & place machine. But Viasion is a flex PCB assembly manufacturer with more than 15 years of experience. We can assemble 1-8 layer flex PCBs, multi-layer flexible PCBs, heavy copper flex circuit boards, and rigid-flex printed circuit boards. Please contact us any time for a free flex PCB assembly quote or flex PCB quote.

    As one of the best flex PCB assembly suppliers in China, Viasion supplies many kinds of flexible PCB assembly, such as 1-8 layer flex PCB, heavy copper flex circuit boards, multi-layer flexible PCBs, rigid-flex PCB, and prototype flex circuits.

    Viasion has more than 15 years of experience as a flex circuits manufacturer; here are our thoughts: 1) flex PCB designs should work well; 2) a trusted flex PCB supplier with experienced engineering, production, purchasing and quality team is extremely important. If you have such demands, please contact us for a free flex PCB assembly quote. 

    The following are the main processes: 1) flexible PCB manufacturing; 2) flex PCB assembly services, which include flex PCB SMT assembly services and flex PCB PTH assembly services; 3) Quality inspection such as AOI inspection, ICT testing, visual inspection and functional test.